LazMapViewer: Fix CanClick in TCustomMarkerClickPlugin

@wpam

With the current condition, the cursor moves to the crHandPoint, even when no point is found. Fix:

 function TCustomMarkerClickPlugin.CanClick(AMapView: TMapView;
   APoint: TGPSPoint): Boolean;
 begin
-  Result := true;
+  Result := Assigned(APoint);
   if Assigned(FOnCanClick) and Assigned(APoint) then
     FOnCanClick(AMapView, APoint, Result);
 end;